The part with the open loop at around 2-oclock will pull out, and is on splines, so you can rotate it and put it back. I think you'll find that is what is acting as the limit of rotation, and therefore how much hot can be added into the mix. http://cdn08.grohe.com/lib/1/tpi/1288624.pdf

Note - Just for the record, I am NOT a plumber and this worked for me.
If you try it and destroy your house, bathroom, shower, yourself or your city - tough luck, you are warned:

Ok - I figured it out. Not sure it is the correct way but it works.

first - I removed the handle attachment thing:

Next - pull out the adapter assembly, first pull on the back ring with the square protrusion at 1-2 o'clock:
This piece can act as a limiter on the handle turning, but I found it is not limiting mine, as you will see further down

unscrew 2 small screws in the adapter and remove it:

now after much suspense and tension, we are at the main act, we have exposed the "stop ring". this piece looks like a brass goldfish.
this image shows it slightly pulled out:

As the handle turns the center spindle - the goldfish turns as well, rotating about 270 degrees counter clockwise
until it's tail hits the metal tab sticking out in the top center. turn it clockwise and the goldfish moves around until it hits the tab from the other side,
and the water stops.

now , as originally set up, there is maybe 10-15 degrees of initial rotation where the spindle turns, but the water flow does not start.
with your fingers, turn the goldfish, still on the spindle, 10 degrees from the top (off position), counter clockwise. the idea is to reset the goldfish so you can increase the hot water mix. if water starts dripping, back off until it stops.
now pull out the goldfish, don't change the spindle, put the goldfish back on so it lines up with the stop tab. i.e. pull it off, rotate the goldfish about 10 degrees back clockwise, and slip it back on, so the tail of the fish is almost touching the tab on top - like what it looked like when first disassembled
You are now good to go - reassemble everything and your water should be hotter: